Recently, disaster recovery of a storage system has drawn attention. A known technique for realizing the disaster recovery is a remote copy system for copying data in a storage apparatus installed at a main site to a storage apparatus installed at a remote site, without sending the data through a host computer.
There are two types of remote copying. One type is synchronous remote copying whereby in synchronization with making update data for a duplication source volume from a host computer reflected in the duplication source volume, the update data is also reflected in a duplication destination volume.
Another type is asynchronous remote copying whereby a buffer for temporarily storing update data is provided at both a main site and a remote site; after storing update data for a duplication source volume from a host computer in the buffer at the main site, the update data is reflected in the duplication source volume; and the update data stored in the buffer for the main site is stored in the buffer for the remote site asynchronously and the pieces of update data stored in the buffer for the remote site are sorted in the update order at the main site and then written to a duplication destination volume. Incidentally, the duplication source volume is a primary volume constituting a copy pair with the duplication destination volume, and the duplication destination volume is a secondary volume of the copy pair.
If the remote copy system is used and a disaster occurs at the main site, processing executed at the main site can be continuously performed at the remote site by using data in the storage apparatus installed at the remote site. Furthermore, a system is suggested for sending data in the storage apparatus at the main site to a third storage apparatus located at a third site and then sending the data in the third storage apparatus located at the third site to the remote site if the main site suffers from a disaster (see Patent Document 1). Incidentally, examples of systems related to this type of technology include: a system for which data consistency between sites is guaranteed (see Patent Document 2); a system for which replica data whose time order is guaranteed is prepared, wherein the system can be restored by using the replica data when a failure has occurred at a remote site (see Patent Document 3); and a system for which the update order of data copied from a main site can be secured among a plurality of secondary volumes at a remote site even if the plurality of secondary volumes at the remote site exist and extend across a plurality of storage subsystems (see Patent Document 4).